Substrate for electronic devices, manufacturing method therefor, and electronic device

ABSTRACT

There is provided a substrate for electronic devices, in which treatment for forming a reconstructed surface or a hydrogen-terminated surface on a substrate is not necessary, and a buffer layer formed on the substrate can be epitaxially grown in the (100) orientation, and a manufacturing method therefor. The substrate  100  for electronic devices comprises; a substrate  11  consisting of silicon, and a first buffer layer  12  and a second buffer layer  13  having a fluorite structure, a first oxide electrode layer  14  having a layered perovskite structure, and a second oxide electrode layer  15  having a simple perovskite structure, which are epitaxially grown and laminated in this order on a film-forming surface of the substrate  11 . The first buffer layer  12  is grown epitaxially at a higher rate than the growth rate of SiO 2 , by irradiating a metallic plasma onto a natural oxide film in an SiO sublimation area.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a substrate for electronic devices, amanufacturing method therefor, and an electronic device. Morespecifically, the substrate for electronic devices according to thepresent invention is preferably used for mounting thereon aferroelectric element functioning as a capacitor, a piezoelectricelement functioning as a cantilever, or the like.

2. Description of the Related Art

Recently, the development of ferroelectric memories, being non-volatilememories using a ferroelectric, is proceeding rapidly. Ferroelectricmemories are divided into; a capacitor type using a ferroelectric as acapacitor, formed in a 1T (transistor)/1C (capacitor) structure, and anMFSFET (Metal Ferroelectric Semiconductor Field Effect Transistor) typeusing a ferroelectric as a gate insulating film for a field-effecttransistor instead of SiO₂. The MFSFET type is more advantageous thanthe capacitor type in view of high integration and non-destructivereading, but it has not yet been realized due to difficulty inmanufacturing related to the structure. Hence, development andcommercialization of the capacitor type is ahead at present.

Representative ferroelectric materials adopted in the capacitor typeferroelectric memory include PbZr_(1-x)Ti_(x)O₃ (abbreviated as PZT) andSrBi₂Ta₂O₉ (abbreviated as SBT). Of these, the PZT having a compositionnear a rhombohedral and tetragonal phase boundary (Moiphotropic PhaseBoundary; abbreviated as MPB) has excellent remanence and anti-fieldcharacteristic, and is put to practical use.

The capacitor type ferroelectric memory has a structure in which theferroelectric material PZT, is placed between a lower electrode and anupper electrode. Pt has heretofore been used as a material forming thelower electrode. Having a face-centered cubic lattice structure, whichis the closest packing structure, Pt has strong self-orientation, and isoriented in a cubic (111) direction even on a thin film having anamorphous structure, such as SiO2, and is hence used preferably.However, since Pt has strong orientation, when columnar crystals grow,there are problems in that Pb or the like is likely to diffuse in thefoundation along the grain boundary, and the bond between Pt and SiO₂becomes poor. As one example of measures against the problems, Ti isused for improving the bond between Pt and SiO₂, and TiN is used forpreventing Pb from diffusing, in many cases. However, when Ti or TiN isused, the lower electrode has a complicated electrode structure, causingoxidation of Ti and diffusion of Ti into Pt, accompanied withdeterioration in crystallinity of PZT. As a result, deterioration inpolarization-electric field (P-E) characteristic, leak currentcharacteristic and fatigue characteristic (tolerance to repeated write)may be caused.

In order to avoid various problems when Pt is used as the lowerelectrode, research for using conductive oxides, as represented byRuO_(x) and IrO₂, for the lower electrode material has been conducted.Among these materials, SrRuO₃ having a perovskite structure has the samecrystalline structure as that of PZT, and hence has excellentbondability on the interface and excellent characteristic as a diffusionbarrier layer for Pb, and can easily realize the epitaxial growth ofPZT. Therefore research into ferroelectric capacitors using SrRuO₃ forthe lower electrode is being actively conducted.

However, in the case of the ferroelectric capacitor having aconstruction where an oxide having a perovskite structure, such asSrRuO₃, is used for the lower electrode, and PZT is provided thereon asa ferroelectric, there are problems as described below.

It is important for PZT to have a composition with more Ti than MPBwhich has a composition of Zr:Ti=0.52:0.48, for example, a compositionof Zr:Ti=0.3:0.7, from a standpoint of an increase in remanence P_(r)and a decrease in the anti-electric field E_(c). However, PZT in thiscomposition range exhibits a tetragonal, and the polarization directionthereof is parallel with the C-axis. As a result, in a ferroelectriccapacitor having a structure in which a lower electrode, a ferroelectricand an upper electrode are laminated in this order on a substrate, it isnecessary to orient the SrRuO₃ electrode itself, being the lowerelectrode, pseudo-cubically (100), in order to allow the PZT forming theferroelectric layer to be a (001) oriented film.

However, when an electrode consisting of SrRuO₃, being a perovskite typeoxide, is directly deposited on an Si substrate, an SiO₂ layer is formedon the interface therebetween. Hence, it is difficult to grow SrRuO₃epitaxially. Therefore, there has been studied a method in which somekind of buffer layer is grown epitaxially on the Si substratebeforehand, and an SrRuO₃ electrode is grown epitaxially on the bufferlayer (for example, see Patent Document 1).

Here the buffer layer epitaxially grown on the Si substrate includesoxides having a fluorite structure, such as yttria stabilized zirconia(abbreviation: YSZ, Zr_(1-x)Y_(x)O_(2-0.5x)) and CeO₂. These materialshave been reported, for example, in non-patent document 1 for YSZ, andin non-patent document 2 for CeO₂/YSZ.

(Patent Document 1)

-   -   Japanese Unexamined Patent Application, First Publication No.        2001-122698

(Non-Patent Document 1)

-   -   Appl. Phys. Lett., vol. 57 (1990) 1137

(Non-Patent Document 2)

-   -   Appl. Phys. Lett., vol. 64 (1994) 1573

The present inventors have conducted research and development for a casewhere a YSZ buffer layer is used, and the SrRuO₃ electrode is grownepitaxially thereon, and as a result, it has been found that thisstructure has two problems described below.

The first problem is that in order to grow a buffer layer such as YSZand CeO₂ on the Si substrate epitaxially, predetermined surfacetreatment is necessary with respect to a film-forming surface of the Sisubstrate, before deposition of the buffer layer thereon.Conventionally, as this surface treatment method, two methods are wellknown, that is, a method of forming a reconstructed surface, and amethod of forming a hydrogen-terminated surface. For example, atechnique in which a surface of the Si substrate is changed to ahydrogen-terminated surface, by subjecting the surface of the Sisubstrate to a treatment by hydrofluoric acid is disclosed in non-patentdocument 3. Here, the reconstructed surface represents a surface inwhich the periodic structure of the surface has been changed to bulk, byperforming heat treatment in a high temperature and high vacuumatmosphere, so as to allow excessive covalent bonds (dangling bonds) inSi atoms forming the surface layer of the Si substrate to bond with eachother. On the other hand, the hydrogen-terminated surface stands for asurface having a structure in which after an SiO₂ natural oxide filmforming the surface layer of the Si substrate is etched by washing withhydrofluoric acid, the dangling bonds on the surface are terminated withhydrogen in ammonium fluoride solution.

(Non-Patent Document 3)

-   -   Appl. Phys. Lett., vol. 57 (1990) 1137

The second problem is that the orientation direction of SrRuO₃ formed onthe buffer layer changes to the (110) orientation (pseudo-cubic). It iswell-known that when SrRuO₃ having a simple perovskite structure (inorthorhombic, a=0.5567 nm, b=0.5530 nm, c=0.7845 nm, and inpseudo-cubic, a=0.3923 nm, 2 ^(1/2)a=0.5548 nm) is provided on the (100)surface of YSZ (a=0.514 nm) or CeO₂ (a=0.541 nm) having a fluoritestructure, epitaxial growth does not occur in the (100) orientation, butis (110) oriented (pseudo-cubically) (for example, see non-patentdocument 4)

(Non-Patent Document 4)

-   -   Appl. Phys. Lett., vol. 67 (1995) 1387

In other words, in order to epitaxially grow the buffer layer such asYSZ or CeO₂ on the Si substrate, predetermined surface treatment hasheretofore been required with respect to the film-forming surface of theSi substrate, before deposition of the buffer layer thereon, therebycausing complication of the manufacturing process and an increase in themanufacturing cost. Even if such predetermined surface treatment isconducted, the epitaxial growth of the buffer layer formed thereoncannot be realized in the desired (100) orientation, but only theepitaxial growth in the (110) orientation can be realized.

SUMMARY OF THE INVENTION

In view of the above situation, it is an object of the present inventionto provide a substrate for electronic devices, in which when a bufferlayer such as YSZ and CeO₂ is grown epitaxially on an Si substrate,predetermined surface treatment, that is, treatment for forming areconstructed surface or treatment for forming a hydrogen-terminatedsurface is not necessary with respect to the film-forming surface of theSi substrate, before deposition of the buffer layer thereon, and even ifthe predetermined surface treatment is not performed, the buffer layercan be epitaxially grown in the (100) orientation, and a manufacturingmethod therefor.

In order to solve the above problems, the present invention provides asubstrate for electronic devices comprising: a substrate consisting ofsilicon, and a first buffer layer, a second buffer layer, a first oxideelectrode layer and a second oxide electrode layer, which are grownepitaxially and laminated in this order on the film-forming surface ofthe substrate, wherein the first buffer layer is a first metal oxidehaving a fluorite structure, the second buffer layer is a second metaloxide having a fluorite structure, the first oxide electrode layer is athird metal oxide having a layered perovskite structure, and the secondoxide electrode layer is a fourth metal oxide having a simple perovskitestructure.

Moreover, the present invention provides a manufacturing method for thesubstrate for electronic devices, comprising:

-   -   a pretreatment step for washing a substrate consisting of        silicon;    -   a first film-forming step in which the substrate having been        subjected to the pretreatment step is arranged in a film-forming        container under a reduced-pressure atmosphere, and a        predetermined gas or plasma is irradiated onto the film-forming        surface of the substrate, to epitaxially grow a first buffer        layer comprising a first metal oxide having a fluorite        structure;    -   a second film-forming step in which a predetermined gas or        plasma is irradiated onto the surface of the first buffer layer,        to epitaxially grow a second buffer layer comprising a second        metal oxide having a fluorite structure;    -   a third film-forming step in which a predetermined gas or plasma        is irradiated onto the surface of the second buffer layer, to        epitaxially grow a first oxide electrode layer comprising a        third metal oxide having a layered perovskite structure; and    -   a fourth film-forming step in which a predetermined gas or        plasma is irradiated onto the surface of the first oxide        electrode layer, to epitaxially grow a second oxide electrode        layer comprising a fourth metal oxide having a simple perovskite        structure.

As described above, according to the present invention, there isprovided a substrate for electronic devices having a construction suchthat the first buffer layer and the second buffer layer having thefluorite structure are grown epitaxially in the cubic (100) orientationon the Si substrate, the first oxide electrode layer having the layeredperovskite structure is grown epitaxially in a tetragonal ororthorhombic (001) orientation thereon, and the second oxide electrodelayer having the simple perovskite structure is grown epitaxially in acubic or pseudo-cubic (100) orientation thereon.

In the substrate for electronic devices, since the outermost surface isthe (100) oriented second oxide electrode layer, when a ferroelectriclayer or a piezoelectric layer having the perovskite structure is formedthereon, the epitaxial growth of the ferroelectric layer or thepiezoelectric layer can be realized easily in the tetragonal (001)orientation or a rhombohedral (100) orientation.

Consequently, using the substrate for electronic devices according tothe present invention, electronic devices provided with functionalelements such as a ferroelectric memory or a piezoelectric elementformed by providing a ferroelectric layer or a piezoelectric layer onthe substrate, can be made with an optimal construction.

According to the manufacturing method of the present invention, when thefirst buffer layer having the fluorite structure is formed on the Sisubstrate, the first buffer layer is grown epitaxially at a higher ratethan the growth rate of SiO₂, by irradiating a metallic element gas orplasma onto the Si substrate coated with a natural oxide film in an SiOsublimation area, so that the first buffer layer can be formed, withoutforming a thermally oxidized amorphous layer between the Si substrateand the first buffer layer. As a result, the first buffer layer having adesired oriented plane and excellent crystallinity can be formed,thereby enabling the second buffer layer, the first oxide electrodelayer and the second oxide electrode layer formed on the first bufferlayer to have excellent crystallinity. As a result, a substrate forelectronic devices enabling production of high performance electronicdevices can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic sectional view illustrating a configuration of asubstrate for electronic devices according to the present invention.

FIGS. 2A to 2E are sectional views in each manufacturing step, when thesubstrate for electronic devices according to the present invention ismanufactured.

FIGS. 3A to 3E are photographs showing diffraction patterns obtained byobserving the outermost surface in each manufacturing step, when thesubstrate for electronic devices according to the present invention ismanufactured, using an RHEED method.

FIGS. 4A and 4B are graphs illustrating results of studying theoutermost surface of the substrate for electronic devices according tothe present invention by an X-ray diffraction method.

FIG. 5 is a sectional view schematically showing one example of afunctional element constituting an electronic device according to thepresent invention.

FIG. 6 is a sectional view schematically showing another example of afunctional element constituting an electronic device according to thepresent invention.

FIG. 7 is a flowchart for each step constituting a manufacturing methodfor a substrate for electronic devices according to the presentinvention.

FIG. 8 is a schematic sectional view showing one example of afilm-forming apparatus used for forming an oxide film constituting thesubstrate for electronic devices according to the present invention.

FIG. 9 is a schematic sectional view illustrating an outline for forminga first buffer layer on the substrate, on the film-forming surface ofwhich an oxide layer exists.

FIG. 10 is a schematic perspective view illustrating one example of afluorite structure.

FIG. 11 is a schematic perspective view illustrating one example of asimple perovskite structure.

FIG. 12 is a schematic perspective view illustrating one example of alayered perovskite structure.

FIG. 13 is a schematic perspective view illustrating another example ofa layered perovskite structure.

FIG. 14 is a graph illustrating a relation between an deposition rate ofthe first buffer layer and a peak half width of YSZ (200).

DETAILED DESCRIPTION OF THE INVENTION

The substrate for electronic devices according to the present inventionwill be described in detail below, with reference to the drawings.

FIG. 1 is a schematic sectional view of the substrate for electronicdevices according to the present invention. In FIG. 1, the substrate 100for electronic devices comprises a substrate 11 consisting of silicon,and a first buffer layer 12, a second buffer layer 13, a first oxideelectrode layer 14 and a second oxide electrode layer epitaxially grownand laminated in this order on a film-forming surface of the substrate11. The first buffer layer 12 is a first metal oxide having a fluoritestructure, the second buffer layer 13 is a second metal oxide having afluorite structure, the first oxide electrode layer 14 is a third metaloxide having a layered perovskite structure, and the second oxideelectrode layer 15 is a fourth metal oxide having a simple perovskitestructure.

FIGS. 2A to 2E are schematic sectional views illustrating states inwhich the first buffer layer 12, the second buffer layer 13, the firstoxide electrode layer 14 and the second oxide electrode layer 15 aresequentially laminated on the substrate 11, when the substrate forelectronic devices in FIG. 1 is manufactured. FIG. 2A illustrates thesubstrate 11 immediately before the first buffer layer 12 is provided,indicating that an oxide layer 16 exists on the film-forming surface.FIG. 2B represents the state where the first buffer layer 12 isprovided, FIG. 2C represents the state where second buffer layer 13 isprovided, FIG. 2D represents the state where the first oxide electrodelayer 14 is provided, and FIG. 2E represents the state where the secondoxide electrode layer 15 is provided.

In the case of the substrate 100 for electronic devices having the aboveconfiguration, the first metal oxide having the fluorite structure isfirst provided as the first buffer layer 12, on the film-forming surfaceof the substrate 11 consisting of silicon. So long as this is the firstmetal oxide having the fluorite structure, then even if the film-formingsurface of the substrate 11 consisting of silicon has not been subjectedto predetermined surface treatment, that is, the treatment for forming areconstructed surface or the treatment for forming a hydrogen-terminatedsurface, epitaxial growth of the first buffer layer 12 comprising thefirst metal oxide becomes possible.

This is because since the predetermined surface treatment has not beenconducted with respect to the film-forming surface of the substrate 11,oxygen remains on the film-forming surface of the substrate 11, with thefilm-forming surface of the substrate 11 covered with SiO₂, and onadherence of the metal oxide thereon, oxygen disengages from thefilm-forming surface of the substrate 11 in the form of SiO, and as aresult, the first buffer layer 12 grows epitaxially.

FIG. 9 is a schematic sectional view illustrating this situation. InFIG. 9, the left figure represents the state in which oxygen remains onthe film-forming surface of the substrate 11, with the film-formingsurface of the substrate 11 covered with SiO₂. In FIG. 9, the rightfigure represents the state in which on adherence of the metal oxidethereon, oxygen disengages from the film-forming surface of thesubstrate 11 in the form of SiO, and as a result, YSZ grows epitaxiallyas the first buffer layer 12.

The present inventors have found by experiments that the material bywhich the above phenomenon can be confirmed is a metal oxide having thefluorite structure. For example, the fluorite structure is as shown inFIG. 10. In this application, the metal oxide forming the first bufferlayer 12 is referred to as a first metal oxide. It has been confirmed byX-ray photoelectron spectroscopy (abbreviated as XPS) that oxygenremains on the film-forming surface of the substrate 11, which has notundergone the predetermined surface treatment.

The present inventors have confirmed by experiments that the abovephenomenon occurs when the orientation of the film-forming surface ofthe substrate 11 consisting of silicon is (100), (110) or (111).

Moreover, they found that the film-forming surface of the substrate 11consisting of silicon when the above phenomenon occurs, that is, thefilm-forming surface that has not been subjected to the predeterminedsurface treatment, such as the treatment for forming a reconstructedsurface or the treatment for forming a hydrogen-terminated surface, canbe specified, since a diffraction pattern is not observed in adiffraction image by the RHEED method, before forming the first bufferlayer.

FIGS. 3A to 3E are photographs of diffraction images observed by theRHEED method. FIG. 3A shows the result of observing the surface wherethe oxide layer 16 exists on the substrate 11, immediately before thefirst buffer layer 12 is formed. FIG. 3B to FIG. 3E respectively showthe results of observation of the surfaces of the first buffer layer 12,the second buffer layer 13, the first oxide electrode layer 14, and thesecond oxide electrode layer 15.

From FIG. 3A, it is clearly seen that when observing the surface onwhich the oxide layer 16 exists on the substrate 11 immediately beforeproviding the first buffer layer 12, the diffraction pattern is notobserved. When the first buffer layer 12, the second buffer layer 13,the first oxide electrode layer 14, and the second oxide electrode layer15 are sequentially laminated on the film-forming surface of thesubstrate in such a condition, the diffraction pattern can be observedgradually becoming distinct in the diffraction image by the reflectionhigh energy electron diffraction (abbreviated as RHEED) method. Theresult of observation indicates the epitaxial growth of each layerformed by the sequential lamination.

In other words, on the epitaxially grown first buffer layer 12, thesecond buffer layer 13 comprising the second metal oxide having thefluorite structure, the first oxide electrode layer 14 comprising thethird metal oxide having the layered perovskite structure, and thesecond oxide electrode layer 15 comprising the fourth metal oxide havingthe simple perovskite structure can be each grown epitaxially.

It becomes clear that if the layered perovskite structure is formed onthe fluorite structure, and then the simple perovskite structure isprovided on the layered perovskite structure, the second oxide electrodelayer 15 laminated last can epitaxially grow in the (100) orientation,which has been a difficult orientation.

FIG. 11 illustrates an example of the simple perovskite structure, andFIG. 12 and FIG. 13 illustrate examples of the layered perovskitestructure.

Since the substrate for electronic devices having the aboveconfiguration comprises an oxide electrode layer epitaxially grown inthe (100) orientation on the outermost surface, the ferroelectricmaterial comprising PZT or SBT can be grown as a (001) oriented filmthereon. Therefore, the substrate for electronic devices according tothe present invention is suitable as a substrate for manufacturingferroelectric elements and piezoelectric elements.

The first metal oxide constituting the first buffer layer 12 is, forexample, a solid solution expressed as Zr_(1-x)Mα_(x)O_(y) (0<x<1,1.5<y<2) obtained by substituting a part of Zr, being a constituentelement of zirconia, by a metal element Mα (where Mα indicates one kindof element selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu. Y, Mg, Ca, Sr and Ba), and is preferably cubically orientedin the (100) direction.

The second metal oxide constituting the second buffer layer 13 is ceriumoxide or a solid solution expressed as Ce_(1-x)Mβ_(x)O_(y) (0<x<1,1.5<y<2) obtained by substituting a part of Ce, being a constituentelement of cerium oxide, by a metal element Mβ (where Mβ indicates onekind of element selected from Zr, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy,Ho, Er, Tm, Yb, Lu, Y, Mg, Ca, Sr and Ba), and is preferably cubicallyoriented in the (100) direction.

The third metal oxide constituting the first oxide electrode layer 14 isa solid solution containing a metal element My or RE as a constituentelement, and expressed as MγRuO₄, RE₂NiO₄, or REBa₂Cu₃Ox (where Mγindicates one kind of element selected from Ca, Sr and Ba, and REindicates one kind of element selected from La, Ce, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and Y), and is preferably tetragonallyor orthorhombically oriented in the (001) direction.

The fourth metal oxide constituting the second oxide electrode layer 15is a solid solution containing a metal element Mγ or RE as a constituentelement, and expressed as MγRuO₃, (RE, Mγ)CrO₃, (RE, Mγ)MnO₃, (RE,Mγ)CoO₃, or (RE, Mγ)NiO₃ (where Mγ indicates one kind of elementselected from Ca, Sr and Ba, and RE indicates one kind of elementselected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,Lu, and Y), and is preferably cubically or pseudo-cubically oriented inthe (100) direction.

FIG. 7 is a flowchart for each step constituting a manufacturing methodfor a substrate for electronic devices according to the presentinvention.

The manufacturing method for a substrate for electronic devicesaccording to the present invention comprises: a pretreatment step forwashing the substrate 11 consisting of silicon (in FIG. 7, expressed as“Si substrate washing step”);

-   -   a first film-forming step in which the substrate 11 having been        subjected to the pretreatment step is arranged in a film-forming        container under a reduced-pressure atmosphere, and a        predetermined gas or plasma is irradiated onto the film-forming        surface of the substrate 11, to epitaxially grow the first        buffer layer 12 comprising the first metal oxide having the        fluorite structure (in FIG. 7, expressed as “epitaxial growth        process of first buffer layer”);    -   a second film-forming step in which a predetermined gas or        plasma is irradiated onto the surface of the first buffer layer        12, to epitaxially grow the second buffer layer 13 comprising        the second metal oxide having a fluorite structure (in FIG. 7,        expressed as “epitaxial growth process of second buffer layer”);    -   a third film-forming step in which a predetermined gas or plasma        is irradiated onto the surface of the second buffer layer 13, to        epitaxially grow the first oxide electrode layer 14 comprising        the third metal oxide having the layered perovskite structure        (in FIG. 7, expressed as “epitaxial growth process of first        oxide electrode layer”); and    -   a fourth film-forming step in which a predetermined gas or        plasma is irradiated onto the surface of the first oxide        electrode layer 14, to epitaxially grow the second oxide        electrode layer 15 comprising the fourth metal oxide having the        simple perovskite structure (in FIG. 7, expressed as “epitaxial        growth process of second oxide electrode layer”).

In the manufacturing method for the substrate for electronic devices, afilm-forming apparatus shown in FIG. 8 is preferably used for themanufacturing apparatus which forms the respective oxide films in thefirst to the fourth film-forming steps. FIG. 8 is a schematic sectionalview of the film-forming apparatus which forms a thin film by a pulsedlaser deposition (PLD) method, wherein reference symbol 80 denotes aprocess chamber, whose inside can be decompressed, 81 denotes arotatable base metal supporting section having film-forming base metalsplaced thereon, referred to as targets, 82 denotes a target placed onthe base metal supporting section 81, 83 denotes a substrate holdingsection, 84 denotes a temperature control section built into thesubstrate holding section 83, 85 denotes a substrate, 86 denotes a thinfilm deposited on the substrate 85, 87 denotes an introduction pipe fora process gas, 88 denotes an opening and closing mechanism forcontrolling the process gas, 89 denotes an (ArF or KrF) excimer laserbeam irradiated onto the film-forming base metal 82, 90 denotes a plume(plasma column) generated by irradiating the excimer laser beam onto thefilm-forming base metal 82, 91 denotes an exhaust section whichdecompresses the inside of the process chamber 80, 92 denotes an openingand closing mechanism which shuts off the inside space of the processchamber 80 from the inside space of the exhaust section 91, 93 denotes aRHEED source (RHEED gun) used at the time of analyzing the thin film bythe RHEED method, 94 denotes a beam shone onto the thin film 86 from theRHEED source 93, 95 denotes a beam reflected by the thin film 86, and 96denotes a mechanism which detects the reflected beam 95, referred to asa RHEED screen.

The PLD method used in the film-forming apparatus shown in FIG. 8 is afilm-forming method in which an excimer laser beam is irradiated onto arotating target in the form of pulses, in an oxygen atmosphere in whichthe inside space of the process chamber 80 is kept at a very lowpressure, for example, at a pressure of about 1/1000 of atmosphericpressure, while forming the thin film 86 on the substrate 85, so thatthe component constituting the target is made fly to the substrate in aplasma or molecular state by this irradiation, thereby depositing thethin film on the film-forming surface of the substrate.

In the manufacturing method for the substrate for electronic deviceshaving this configuration, the pretreatment step stands for washingtreatment for removing organics from the surface layer of the substrate11 consisting of silicon, and drying treatment conducted thereafter. Inthis washing treatment, for example, the substrate 11 is soaked in anorganic solvent, or ultrasonic waves are applied simultaneously withsoaking, to remove organics adhered to the substrate 11. The dryingtreatment is for removing the organic solvent used in the washingtreatment, from the film-forming surface of the substrate 11.

At this time, in this pretreatment step, it is desired to finish thefilm-forming surface of the substrate 11 as a surface where the oxidelayer 16 exists, not as a reconstructed surface or a hydrogen-terminatedsurface. Specifically, the RCA washing or washing with hydrofluoric acidfor obtaining the reconstructed surface or the hydrogen-terminatedsurface, which has been heretofore conducted after the washing treatmentusing the organic solvent, is not conducted, so as to finish thefilm-forming surface of the substrate 11 in a state with oxygen (oxidelayer 16) remaining thereon (FIG. 2A).

In the first film-forming step, the substrate 11 having undergone thepretreatment step is arranged in a film-forming container under areduced-pressure atmosphere, and a predetermined gas or plasma isirradiated onto the film-forming surface of the substrate 11, toepitaxially grow the first buffer layer 12 comprising the first metaloxide having the fluorite structure. Here the predetermined gas orplasma comprises an element constituting the metal oxide to be formed inthe first film-forming step, and this is irradiated onto thefilm-forming surface of the substrate 11.

Oxygen (the oxide layer 16) remains on the film-forming surface of thesubstrate 11 produced in the pretreatment step, and the film-formingsurface is covered with SiO₂. Therefore, when this film-forming surfaceis exposed to the gas or plasma comprising the element constituting themetal oxide, the element adheres to the film-forming surface, so thatthe metal oxide starts to deposite, and at the same time, the SiO₂ filmforming the oxide layer 16 existing on the film-forming surface issublimated in the form of SiO and removed from the film-forming surfacestruck by the gas or plasma. At the time of removing SiO, the substrate11 is held under a temperature, a back pressure, and an oxygen partialpressure such that the vapor pressure of SiO does not reach thesaturated vapor pressure, and the deposition rate of the first bufferlayer 12 is set to not lower than a growth rate of the thermallyoxidized SiO₂ film on the film-forming surface of the substrate 11.

By this action, a silicon oxide hardly exists on the interface betweenthe substrate 11 consisting of silicon and the first buffer layer 12, sothat the epitaxially grown first buffer layer 12 can be provideddirectly on the film-forming surface of the substrate 11 (FIG. 2B). Themetal oxide having the fluorite structure is preferably used for thefirst buffer layer, since the above action can be stably obtained.

When the first buffer layer 12 is formed, the first film-forming step iscarried out under film-forming conditions such that the temperature ofthe substrate 11 is 800° C. or lower, the back pressure in thefilm-forming atmosphere is not lower than 1×10⁻⁶ Torr, and the oxygenpartial pressure in the film-forming atmosphere is not lower than 5×10⁻⁶Torr, and more preferably, under film-forming conditions such that thetemperature of the substrate 11 is from 600° C. to 800° C. inclusive,the back pressure in the film-forming atmosphere is from 1×10⁻⁶ Torr to5×10⁻⁶ Torr inclusive, and the oxygen partial pressure in thefilm-forming atmosphere is from 5×10⁻⁶ Torr to 5×10⁻⁴ Torr inclusive (1Torr=133 Pa).

Moreover, the deposition rate of the first buffer layer 12 is preferablyset to not lower than the growth rate (0.2 nm/min.) of the thermallyoxidized SiO₂ film. One example of the film-forming conditions forobtaining this deposition rate is such that the density of the laserenergy is not lower than 1 J/cm², the laser frequency is not lower than5 Hz, and the target-substrate distance is from 50 mm to 80 mminclusive.

Here, it is important to set the deposition rate of the first bufferlayer 12 to not lower than the growth rate (0.2 nm/min.) of thethermally oxidized SiO₂ film. This is because, from a verificationexperiment by the present inventors relating to the relation between thedeposition rate and the crystallinity of the first buffer layer, it wasfound that when the deposition rate was lower than 0.2 nm/min, which isthe growth rate of the thermally oxidized SiO₂ film, the epitaxialgrowth of the first buffer layer 12 could not be achieved. The detailsof the verification experiment are described below.

The experiment was conducted by preparing a pretreated Si substrate, andforming a YSZ film on the Si substrate as the first buffer layer by thePLD method using the film-forming apparatus shown in FIG. 8. At thistime, the deposition rate of YSZ was adjusted by variously changing thelaser frequency, the target-substrate distance and the laser energydensity. The respective experiment conditions, the deposition rate ofYSZ, and the full width at half maximum (FWHM) in a rocking curve of aYSZ (200) peak obtained by measuring the formed YSZ by the XRD are shownin Table 1.

The substrate temperature at the time of film forming was 700° C., theback pressure in the film-forming atmosphere was 1×10⁻⁶ Torr, and theoxygen partial pressure was 5×10⁻⁵ Torr, which were common in allconditions.

FIG. 14 is a graph in which the results shown in Table 1 are itemizedfor each parameter used for the adjustment of the deposition rate. Inthis graph, the deposition rate of YSZ is plotted on the X axis, and thepeak half width of YSZ (200) is plotted on the Y axis. As is obviousfrom the results shown in Table 1 and FIG. 14, in the range where thedeposition rate is lower than 0.2 nm/min. (1 nm/5 min.), the peak halfwidth of YSZ (200) is extremely large, indicating that YSZ has notepitaxially grown. On the contrary, in the range where the depositionrate is not lower than 0.2 nm/min., it is seen that the peak half widthbecomes narrow, and YSZ has epitaxially grown favorably.

Moreover, as shown in FIG. 14, since the relation between the depositionrate and the peak half width do not rely on the parameters used for theadjustment of the deposition rate, it is suggested that the reactionwhich inhibits the epitaxial growth of YSZ on the Si substrateprogresses on the Si substrate. This deposition rate of 0.2 nm/min.substantially agrees with the deposition rate of the thermally oxidizedSiO₂ film on the Si substrate, and the deposition process of YSZ isconducted in an oxygen-containing atmosphere by heating the substrate toa high temperature. Therefore, it can be presumed that the reactioninhibiting the epitaxial growth of YSZ is a reaction for forming thethermally oxidized SiO₂ film on the surface of the Si substrate.

From the above results, in the first film-forming step for forming thefirst buffer layer 12, it can be said that in order that the firstbuffer layer 12 is grown epitaxially, it is necessary to set thedeposition rate thereof faster than the growth rate of the thermallyoxidized SiO₂ film on the surface of the Si substrate. TABLE I Target-Laser YSZ Laser substrate energy deposition YSZ (200) Sample frequencydistance density rate peak half No. (Hz) (mm) (J/cm²) (nm/5 min.) width(deg.) 1 2 50 1.5 0.58 10 2 5 50 1.5 1.46 2.5 3 10 50 1.5 2.92 1.91 4 1080 1.5 1.25 1.31 5 10 70 1.5 1.67 1.22 6 10 60 1.5 2.08 0.99 7 10 50 1.52.92 1.06 8 10 50 0.5 0.42 10 9 10 50 1.0 1.67 2.29 10 10 50 1.5 2.921.06

In the formation of the first buffer layer 12, since the above describedaction operates, it is not necessary to increase the substratetemperature more than necessary, or set the back pressure in thefilm-forming atmosphere to ultra-high vacuum. On the other hand, in theconventional process, a substrate having a film-forming surfacecomprising a reconstructed surface or a hydrogen-terminated surface isused, and the first buffer layer is formed thereon. In this case, inorder to achieve the epitaxial growth of the first buffer layer, asubstrate temperature exceeding 800° C. and a film-forming atmosphere inwhich the back pressure is lower than 1×10⁻⁶ Torr are the essentialconditions. Therefore, according to the first film-forming step of thepresent invention, the energy required for producing the first bufferlayer 12 can be considerably reduced, thereby contributing to areduction in the production cost.

In the second film-forming step, a predetermined gas or plasma isirradiated onto the surface of the first buffer layer 12 formed in thefirst film-forming step, to epitaxially grow the second buffer layer 13comprising the second metal oxide having the fluorite structure.

For the second buffer layer 13, there is selected one comprising thesecond metal oxide, being a different material from that of the firstmetal oxide constituting the first buffer layer 12, and having an effectof stimulating the epitaxial growth of the first oxide electrode layer14 to be laminated on the second buffer layer 13.

Depending on the film-forming condition, there may be a case in whichdroplet particles having a diameter of from 1 to 10 μm are generated onthe surface of the first buffer layer 12 formed in the firstfilm-forming step. In this case, the droplet particles may be removed byadding a step of wiping off the surface of the first buffer layer 12,using a flexible material or a member having a flexible structure. Forexample, when the droplet particles of about 100 to 1000 pcs./cm existbefore removal, the number thereof can be decreased up to 0.1 to 1pcs/cm² after removal, by using a cotton bud as the above member. Theremoval of the droplet particles generated on the surface of the firstbuffer layer 12 stimulates the epitaxial growth of the second bufferlayer 13 formed thereon, thereby contributing to ensure the flatness.

In the third film-forming step, a predetermined gas or plasma isirradiated onto the surface of the second buffer layer 13 formed in thesecond film-forming step, to epitaxially grow the first oxide electrodelayer 14 comprising the third metal oxide having the layered perovskitestructure.

Finally, in the fourth film-forming step, a predetermined gas or plasmais irradiated onto the surface of the first oxide electrode layer 14formed in the third film-forming step, to epitaxially grow the secondoxide electrode layer 15 comprising the fourth metal oxide having thesimple perovskite structure.

Conventionally, there is used a structure in which the first oxideelectrode layer 14 comprising the third metal oxide having the layeredperovskite structure is not provided, but the second oxide electrodelayer 15 comprising the fourth metal oxide having the simple perovskitestructure is directly formed on the surface of the second buffer layer12. In this conventional structure in which the simple perovskitestructure is directly arranged without providing the layered perovskitestructure, the outermost surface is oriented in the (110) direction.

On the other hand, in the manufacturing method of the substrate forelectronic devices according to the present invention, by adopting thestep where the simple perovskite structure is formed on the layeredperovskite structure, an oxide electrode layer epitaxially grown in the(100) orientation can be formed on the outermost surface.

For the first oxide electrode layer 14, there is selected one comprisingthe third metal oxide, being a different material from that of thesecond metal oxide constituting the second buffer layer 13, and havingan effect of stimulating the epitaxial growth of the second oxideelectrode layer 15 to be laminated on the first oxide electrode layer14.

For the second oxide electrode layer 15, one that epitaxially grows onthe first oxide electrode layer 14, and is oriented in the (100)direction is selected.

In the aforementioned manufacturing method for a substrate forelectronic devices, in each step of the first to the fourth film-formingsteps, a predetermined gas or plasma comprising an element constitutingthe metal oxide to be formed in each step is used.

Particularly, in the first film-forming step, when a gas or plasmagenerated by irradiating a laser beam onto a base metal arrangedopposite to the film-forming surface of the substrate 11 is used as thepredetermined gas or plasma, it is desired to irradiate the gas orplasma of the constituent element of the first buffer layer 12 onto thesubstrate 11 held under a temperature, a back pressure and an oxygenpartial pressure such that the vapor pressure of SiO does not reach thesaturated vapor pressure, so that an SiO₂ film forming the oxide layer16 existing on the film-forming surface of the substrate 11 is reducedto Si. Moreover, it is desired that the first buffer layer 12 isdeposited on the film-forming surface at an deposition rate not lowerthan the rate at which the thermally oxidized SiO₂ film is formed, whilesublimating and removing the Si formed by reduction as SiO, so that thefirst buffer layer 12 is epitaxially grown, without forming an amorphouslayer on the interface between the substrate 11 and the first bufferlayer 12. If formed in this manner, the first buffer layer 12 having adesired orientation plane and excellent crystallinity can be formed, ina simple manufacturing process, which is also desirable from astandpoint of reducing the manufacturing cost.

The electronic device according to the present invention is a functionalelement furnished with the substrate for electronic devices having theabove described configuration.

In other words, in the substrate for electronic devices having the abovedescribed configuration, since the outermost surface comprises an oxideelectrode layer epitaxially grown in the (100) orientation, then when aferroelectric layer constituting a functional element represented by acapacitor (ferroelectric element) or a cantilever (piezoelectricelement) is directly provided thereon, the ferroelectric layer canachieve excellent crystal growth. Therefore, since desiredcharacteristics can be stably obtained, as compared with theconventional method, by using a functional element furnished with thesubstrate for electronic devices according to the present invention,then this is preferable.

FIG. 5 is a schematic sectional view illustrating one example of afunctional element according to the present invention, wherein thefunctional element is a ferroelectric element (ferroelectric capacitor)200. In FIG. 5, reference symbol 21 denotes a substrate consisting ofsilicon, 22 denotes a first buffer layer, 23 denotes a second bufferlayer, 24 denotes a first lower electrode layer, 25 denotes a secondlower electrode layer, 26 denotes a ferroelectric layer, and 27 denotesan upper electrode layer, with 21 to 25 corresponding to the abovedescribed substrate 100 for electronic devices.

FIG. 6 is a schematic sectional view illustrating another example of thefunctional element according to the present invention, wherein thefunctional element is a piezoelectric element (piezoelectric actuator)300. In FIG. 6, reference symbol 31 denotes a substrate consisting ofsilicon, 32 denotes a first buffer layer, 33 denotes a second bufferlayer, 34 denotes a first lower electrode layer, 35 denotes a secondlower electrode layer, 36 denotes a piezoelectric layer, 37 denotes anupper electrode layer, and 38 denotes a thermal oxide film layer, with31 to 35 including 38 corresponding to the above described substrate 100for electronic devices.

EXAMPLES

The present invention will be described in detail by way of examples,but the present invention is not limited to these examples.

Example 1

In this example, a specific example of the substrate for electronicdevices having a configuration in which a first buffer layer 12, asecond buffer layer 13, a first oxide electrode layer 14 and a secondoxide electrode layer 15 are sequentially laminated on a siliconsubstrate 11, with a film-forming surface being a (100) plane, and themanufacturing method therefor will be described, with reference to FIG.1 to FIG. 4.

FIG. 1 is a schematic sectional view illustrating the configuration ofthe substrate for electronic devices according to this example, and FIG.2 is a sectional view in each manufacturing step, when the substrate forelectronic devices in FIG. 1 is manufactured. FIGS. 3A to 3E arephotographs showing diffraction patterns obtained by observing theoutermost surface in each manufacturing step, when the substrate forelectronic devices in FIG. 1 is manufactured, using the RHEED method.FIGS. 4A and 4B are graphs illustrating results of studying theoutermost surface of the substrate for electronic devices shown in FIG.1 by an X-ray diffraction (XRD) method, wherein FIG. 4A represents theresult of 0-20 scanning, and 4B represents the result of omega scanningof an SrRuO₃ (200) peak.

At first, the configuration of the substrate for electronic devicesaccording to this embodiment will be described.

The substrate 100 for electronic devices according to this embodimentcomprises an Si substrate comprising a (100) plane, a first buffer layer12 comprising a metal oxide having a fluorite structure epitaxiallygrown in a cubic (100) orientation on the Si substrate 11, a secondbuffer layer 13 comprising a metal oxide having a fluorite structureepitaxially grown in a cubic (100) orientation on the first buffer layer12, a first oxide electrode layer 14 having a perovskite structureepitaxially grown in a tetragonal or orthorhombic (001) orientation onthe second buffer layer 13, and a second oxide electrode layer 15 havinga perovskite structure epitaxially grown in a cubic or pseudo-cubic(100) orientation on the first oxide electrode layer 14.

For the Si substrate 11, a substrate in which the natural oxide film isnot removed is used. In the first buffer layer 12, YSZ is epitaxiallygrown in the cubic (100) orientation to a thickness of 5 nm. In thesecond buffer layer 13, CeO₂ is epitaxially grown in the cubic (100)orientation to a thickness of 10 nm. In the first oxide electrode layer14, YBa₂Cu₃O_(x) is epitaxially grown in the tetragonal or orthorhombic(001) orientation. In the second oxide electrode layer 15, SrRuO₃ isepitaxially grown in the pseudo-cubic (100) orientation.

The manufacturing method for the substrate for electronic devices willnow be described.

The Si (100) substrate 11 is soaked in an organic solvent, anddegreasing wash is conducted, using an ultrasonic cleansing machine. Forthe organic solvent, for example, 1:1 liquid mixture of ethyl alcoholand acetone can be used, but it is not limited thereto. Moreover, it isnot necessary to conduct a step for removing the natural oxide film,such as the RCA washing and washing with hydrofluoric acid, which arenormal representative washing methods for Si substrates. As a result, asshown in FIG. 2A, the natural oxide film is formed on the surface of theSi (100) substrate 11.

After loading the degreased Si (100) substrate 11 in a substrate holder,the substrate holder is introduced into a vacuum device in which theback pressure at room temperature is kept as 1×10⁻⁸ Torr, and the vacuumdevice is heated to warm up to a temperature of 700° C. at a rate of 10°C./min., using an infrared ray lamp. During heating, in a temperaturerange of 500° C. or higher, the natural oxide film layer 16 evaporatespartly as SiO, and hence the pressure increases to 1×10⁻⁶ Torr orhigher, but at a temperature of 700° C., the pressure becomes constantat not higher than 5×10⁻⁷ Torr. However, as shown in FIG. 3A, it is seenthat a diffraction pattern is not observed in the RHEED pattern from anSi <011> direction, and a reconstructed surface is not formed in the Si(100), and hence, the Si substrate is covered with a natural oxide filmlayer 16 shown in FIG. 2A. The conditions of warm-up rate, substratetemperature, pressure and the like are not limited to these, providedthey are within a range that does not form a new thermal oxide film onthe surface of the Si substrate.

After the pressure becomes constant, pulsed radiation of a KrF excimerlaser (wavelength: 248 nm) is shone onto the YSZ target surface arrangedopposite to the Si (100) substrate 11, under conditions of an energydensity of 2.5 J/cm², a frequency of 10 Hz, and a pulse length of 10 ns,to generate a plasma plume consisting of Y, Zr and 0 on the targetsurface. This plasma plume is irradiated onto the Si (100) substrate 11located at a position away from the target by 40 mm, for 10 minutesunder conditions of a substrate temperature of 700° C., and a pressureduring deposition of 5×10⁻⁵ Torr, so that the YSZ first buffer layer 12is deposited to 5 nm (FIG. 2B). As shown in FIG. 3B, a diffractionpattern has appeared in the RHEED pattern from the Si <011> direction,and it is clearly seen that the epitaxial growth is achieved in anazimuthal relation of YSZ (100)/Si (100), YSZ <011>//Si<011>.

Of the respective conditions, it is desired that the target compositionis YSZ, the substrate temperature is from 600° C. to 800° C. inclusive,and the back pressure during deposition is not higher than 5×10⁻⁴ Torr.However, the respective conditions are not limited thereto, so long asthe oxide layer SiO₂ (natural oxide film) on the surface of the Sisubstrate 11 is reduced to Si, by irradiating a Zr gas or plasma ontothe Si substrate 11 held at a temperature, a back pressure and an oxygenpartial pressure such that the vapor pressure of SiO does not reach thesaturated vapor pressure, and so long as YSZ can be grown epitaxiallywhile removing Si by sublimating the Si formed by reduction as SiO. Ofthe respective conditions, it is desired that the laser energy densityis not lower than 1 J/cm², the laser frequency is not lower than 5 Hz,and the target-substrate distance is from 50 mm to 80 mm inclusive.However, the respective conditions are not limited thereto, so long asthe deposition rate of YSZ is not lower than the growth rate (0.2nm/min.) of the thermally oxidized SiO₂ film on the surface of the Sisubstrate, and YSZ can be grown epitaxially without forming thethermally oxidized SiO₂ film on the surface of the Si substrate 11.

However, depending on the conditions, there may be a case where the YSZfirst buffer layer 12 does not change, but oxygen is supplied to theinterface between the Si (100) substrate 11 and the YSZ first bufferlayer 12, to thereby form a thermal oxide film. Moreover, if ZrO₂ formsa solid solution as a cubic, then similar effects can be obtained whenat least one of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,Lu, Mg, Ca, Sr and Ba is added instead of Y.

After having deposited the YSZ first buffer layer 12, pulsed radiationof the KrF excimer laser is shone onto a CeO₂ target surface arrangedopposite to the substrate, under conditions of an energy density of 2.5J/cm², a frequency of 10 Hz, and a pulse length of 10 ns, to generate aplasma plume consisting of Ce and O on the target surface. This plasmaplume is irradiated onto the Si (100) substrate 11 located at a positionaway from the target by 40 mm, for 10 minutes under conditions of asubstrate temperature of 700° C., and a pressure during deposition of5×10⁻⁵ Torr, so that the CeO₂ second buffer layer 13 is deposited to 10nm (FIG. 2C). As shown in FIG. 3C, a diffraction pattern has appeared inthe RHEED pattern from the Si <011> direction, and it is clearly seenthat the epitaxial growth is achieved in an orientation relation of CeO₂(100)/YSZ (100)/Si (100), and in an azimuthal relation of CeO₂<011>//YSZ <011>//Si <011>.

Of the respective conditions, it is desired that the target compositionis CeO₂, the laser energy density is from 2 J/cm to 3 J/cm² inclusive,the laser frequency is not lower than 5 Hz and not higher than 15 Hz,the target-substrate distance is from 30 mm to 50 mm inclusive, thesubstrate temperature is from 650° C. to 750° C. inclusive, and thepressure during deposition is from 1×10⁻⁵ Torr to 1×10⁻⁴ Torr inclusive.However, the respective conditions are not limited thereto, so long asthe epitaxial growth as CeO₂ is possible. Moreover, if CeO₂ forms asolid solution as a cubic, then similar effects can be obtained when atleast one of Zr, La, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu,Mg, Ca, Sr and Ba is added.

After having deposited the CeO₂ second buffer layer 13, pulsed radiationof the KrF excimer laser is shone onto a YBa₂Cu₃O_(x) target surfacearranged opposite to the substrate, under conditions of an energydensity of 2.5 J/cm², a frequency of 5 Hz, and a pulse length of 10 ns,to generate a plasma plume consisting of Y, Ba, Cu and 0 on the targetsurface. This plasma plume is irradiated onto the Si substrate 11located at a position away from the target by 40 mm, for 2 minutes underconditions of a substrate temperature of 600° C., and an oxygen partialpressure during deposition of 1×10² Torr, so that the YBa₂Cu₃O_(x) firstoxide electrode layer 14 is deposited to 2 nm (FIG. 2D).

As shown in FIG. 3D, a clear diffraction pattern has appeared in theRHEED pattern from the Si <011> direction, and it is clearly seen thatthere is an orientation relation of YBa₂Cu₃O_(x) (001)/CeO₂ (100)/YSZ(100)/Si (100), and an azimuthal relation of YBa₂Cu₃O_(x)<100>//CeO₂<011>//YSZ <011>//Si<011>. Of the respective conditions, itis desired that the target composition is YBa₂Cu₃O_(x), the laser energydensity is from 2 J/cm² to 3 J/cm² inclusive, the laser frequency is notlower than 2 Hz and not higher than 10 Hz, the target-substrate distanceis from 30 mm to 50 mm inclusive, the substrate temperature is from 550°C. to 650° C. inclusive, and the pressure during deposition is from1×10⁻³ Torr to 1×10⁻¹ Torr inclusive. However, the respective conditionsare not limited thereto, so long as the epitaxial growth as YBa₂Cu₃O₁ ispossible.

Moreover, similar effects can be obtained by using M₂RuO₄ (M=Ca, Sr orBa), RE₂NiO₄ (RE=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,Lu or Y), or REBa₂Cu₃O (RE=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho,Er, Tm, Yb or Lu), instead of YBa₂Cu₃O_(x).

After having deposited the YBa₂Cu₃O first oxide electrode layer 14,pulsed radiation of the KrF excimer laser is shone onto an SrRuO₃ targetsurface arranged opposite to the substrate, under conditions of anenergy density of 2.5 J/cm², a frequency of 5 Hz, and a pulse length of10 ns, to generate a plasma plume consisting of Sr, Ru and 0 on thetarget surface. This plasma plume is irradiated onto the Si substrate 11located at a position away from the target by 40 mm, for 30 minutesunder conditions of a substrate temperature of 600° C., and an oxygenpartial pressure during deposition of 1×10⁻² Torr, so that the SrRuO₃second oxide electrode layer 15 is deposited to 100 nm (FIG. 2E).

As shown in FIG. 3E, a diffraction pattern has appeared in the RHEEDpattern from the Si <011> direction, and it is clearly seen that thereis an orientation relation of SrRuO₃ (100)/YBa₂Cu₃O_(x) (001)/CeO₂(100)/YSZ (100)/Si (100), and an azimuthal relation ofSrRuO₃<010>//YBa₂Cu₃O_(x) <100>//CeO₂ <011>//YSZ <011>//Si <011>. Of therespective conditions, it is desired that the target composition isSrRuO₃, the laser energy density is from 2 J/cm² to 3 J/cm² inclusive,the laser frequency is not lower than 2 Hz and not higher than 10 Hz,the target-substrate distance is from 30 mm to 50 mm inclusive, thesubstrate temperature is from 550° C. to 650° C. inclusive, and theoxygen partial pressure during deposition is from 1×10⁻³ Torr to 1×10⁻¹Torr inclusive. However, the respective conditions are not limitedthereto, so long as the Sr and Ru plasma can reach the substrate at aconstant ratio of 1:1, and the epitaxial growth as SrRuO₃ is possible.

Moreover, similar effects can be obtained by using MRuO₃ (M=Ca or Ba),(RE M)CrO₃ (RE=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb,Lu or Y; M=Ca, Sr, or Ba), (RE M)MnO₃ (RE=La, Ce, Pr, Nd, Pm, Sm, Eu,Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y; M=Ca, Sr, or Ba), (RE, M)CoO₃(RE=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu or Y;M=Ca, Sr, or Ba), or (RE M)NiO₃ (RE=La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb, Lu or Y), instead of SrRuO₃.

The XRD θ-2θ scan of the thus obtained substrate 100 for electronicdevices is shown in FIG. 4A, wherein peaks of pseudo-cubic SrRuO₃ (100)and SrRuO₃ (200) are clearly observed, together with the peaks of Si(200) and Si (400), which agrees with the observation result by theRHEED, indicating the (100) orientation. An ω scan of the SrRuO₃ (200)peak is shown in FIG. 4B, by which it becomes clear that the full widthat half maximum is 1.9 degrees, the crystallinity is excellent, and itis also useful as a foundation for epitaxial growth of a ferroelectrichaving the perovskite structure. Moreover, measurement of electricalresistivity by the four terminal method makes it clear that a favorablevalue of ρ=550 μΩcm can be obtained at a room temperature, and there isno problem in the basic characteristic as an electrode thin film.

Moreover, after the epitaxial growth process of the first butter layer12, by mechanically brushing off the substrate surface, using a cottonbud or a brush, droplets adhered on the substrate surface at the time ofthe epitaxial growth of the first buffer layer 12 are removed, and byexecuting the epitaxial growth processes of the second buffer layer 13and the like, a clean and flat substrate surface without dropletparticles can be obtained.

Furthermore, if layers up to the first oxide electrode layer 14 haveepitaxially grown, then in the deposition step of the second oxideelectrode layer 15, any film-forming method, such as an applicationmethod by spin coating, dip coating or ink jet printing, a chemicalevaporation method such as MOCVD, and a physical vapor deposition methodsuch as sputtering can be used instead of the laser ablation, to obtainsimilar effects.

According to the substrate for electronic devices having the abovedescribed configuration, a perovskite substrate for electronic devicespseudo-cubically oriented in the (100) direction can be deposited on theSi substrate coated with a natural oxide film, and as a lower electrodefor allowing oxides having the perovskite structure to grow epitaxiallythis enables realization of improvement in the characteristics ofvarious kinds of electronic devices, as represented by the ferroelectriccapacitor.

Example 2

In this example, the ferroelectric capacitor in which the substrate forelectronic devices according to the present invention is used as thelower electrode, whose schematic sectional view is shown in FIG. 5, andthe manufacturing method therefor will be described.

The ferroelectric capacitor 200 according to this example comprises; anSi substrate 21 consisting of a (100) plane, a first buffer layer 22comprising a metal oxide having a fluorite structure epitaxially grownin a cubic (100) orientation on the Si substrate 21, a second bufferlayer 23 comprising a metal oxide having a fluorite structureepitaxially grown in a cubic (100) orientation on the first buffer layer22, a first lower electrode layer 24 having a perovskite structureepitaxially grown in a tetragonal or orthorhombic (001) orientation onthe second buffer layer 23, a second lower electrode layer 25 having aperovskite structure epitaxially grown in a cubic or pseudo-cubic (100)orientation on the first lower electrode layer 24, a ferroelectric layer26 having a perovskite structure epitaxially grown in a tetragonal (001)orientation on the second lower electrode layer 25, and an upperelectrode layer 27 formed on the ferroelectric layer 26.

For the Si substrate 21, a substrate in which the natural oxide film isnot removed is used. In the first buffer layer 22, YSZ is epitaxiallygrown in the cubic (100) orientation to a thickness of 5 nm. In thesecond buffer layer 23, CeO₂ is epitaxially grown in the cubic (100)orientation to a thickness of 10 nm. In the first oxide electrode layer24, YBa₂Cu₃O_(x) is epitaxially grown in the tetragonal or orthorhombic(001) orientation, to a thickness of 2 nm. In the second oxide electrodelayer 25, SrRuO₃ is epitaxially grown in the pseudo-cubic (100)orientation, to a thickness of 100 nm. In the ferroelectric layer 26,PbZr_(0.40)Ti_(0.60)O₃ is epitaxially grown in the tetragonal (001)orientation. Pt is used for the upper electrode layer 27.

The manufacturing method for the ferroelectric capacitor will bedescribed below.

The washing step of the Si (100) substrate 21, the heating and warm-upstep, the deposition step of the first buffer layer 22, the depositionstep of the second buffer layer 23, the deposition step of the firstlower electrode layer 24, and the deposition step of the second lowerelectrode layer 25 are the same as in the manufacturing method for thesubstrate 100 for electronic devices shown in Example 1. After havingdeposited the second lower electrode layer 25, pulsed radiation of theKrF excimer laser is shone onto a PbZr_(0.40)Ti_(0.60)O₃ target surfacearranged opposite to the substrate, under conditions of an energydensity of 2.0 J/cm², a frequency of 5 Hz, and a pulse length of 10 ns,to generate a plasma plume consisting of Pb, Zr, Ti and 0 on the targetsurface. This plasma plume is irradiated onto the Si substrate 21located at a position away from the target by 40 mm, for 30 minutesunder conditions of a substrate temperature of 600° C., and an oxygenpartial pressure during deposition of 1×10⁻² Torr, so that thePbZr_(0.40)Ti_(0.60)O₃ ferroelectric layer 24 is deposited to 150 nm.

Of the respective conditions, it is desired that the target compositionis PZT, the laser energy density is from 1.0 J/cm² to 3.0 J/cm²inclusive, the laser frequency is not lower than 2 Hz and not higherthan 10 Hz, the target-substrate distance is from 30 mm to 50 mminclusive, the substrate temperature is from 550° C. to 650° C.inclusive, and the oxygen partial pressure during deposition is from1×10 ⁻³ Torr to 1×10⁻¹ Torr inclusive. However, the respectiveconditions are not limited thereto, so long as the epitaxial growth asPZT is possible. Moreover, similar effects can be obtained by usingferroelectrics having the perovskite structure, as represented byBaTiO₃, KNbO₃, and BiFeO₃, and solid solutions of these ferroelectricsand various kinds of paraelectrics, instead of PZT.

After having deposited the PZT ferroelectric layer 26, etching isperformed by using a known method such as photolithography, and thesecond lower electrode layer 25 is taken out, and a Pt upper electrodelayer 27 is deposited by a known method such as sputtering, while usinga mask pattern. Similar effects can be obtained by using other electrodematerials generally used for electrodes, such as Ir, instead of Pt.

The thus obtained ferroelectric capacitor 200 has an orientationrelation of Pt/PZT (001)/SrRuO₃ (100)/YBa₂Cu₃O_(x) (001)/CeO₂ (100)/YSZ(100)/Si (100), and an azimuthal relation of PZT<010>//SrRuO_(3<010)>//YBa₂Cu₃O_(x) <100>//CeO₂ <011>//YSZ<011>//Si<011> within a plane.

As a result of P-E hysteresis measurement with respect to the obtainedferroelectric capacitor 200 by applying an electric field having afrequency of I kHz and an amplitude of 100 kV/cm, a remanence P_(r)=90μC/cm² was obtained. This indicates a higher characteristic than Pr=50μC/cm² for the ferroelectric capacitor in which a non-oriented PZTferroelectric layer was used.

Moreover, if layers up to the second lower electrode layer 25 haveepitaxially grown, then in the deposition step of the ferroelectriclayer 26, any film-forming method, such as an application method by spincoating, dip coating or ink jet printing, a chemical evaporation methodsuch as MOCVD, and a physical vapor deposition method such as sputteringcan be used instead of the laser ablation, to obtain similar effects.

According to the ferroelectric capacitor in which the substrate forelectronic devices having the above described configuration is used forthe lower electrode, the tetragonal (001) epitaxial growth of theferroelectric layer having the perovskite structure can be achieved,thereby realizing a ferroelectric memory having a ferroelectriccapacitor excellent in the polarization characteristic.

Example 3

In this example, a piezoelectric actuator in which the substrate forelectronic devices according to the present invention is used for thelower electrode, with the schematic sectional view thereof shown in FIG.6, and a manufacturing method therefor will be described.

The piezoelectric actuator 300 according to this example comprises; anSi substrate 31 consisting of a (100) plane, a first buffer layer 32comprising a metal oxide having a fluorite structure epitaxially grownin a cubic (100) orientation on the Si substrate 31, a second bufferlayer 33 comprising a metal oxide having a fluorite structureepitaxially grown in a cubic (100) orientation on the first buffer layer32, a first lower electrode layer 34 having a perovskite structureepitaxially grown in a tetragonal or orthorhombic (001) orientation onthe second buffer layer 33, a second lower electrode layer 35 having aperovskite structure epitaxially grown in a cubic or pseudo-cubic (100)orientation on the first lower electrode layer 34, a piezoelectric layer36 having a perovskite structure epitaxially grown in a rhombohedral(100) orientation on the second lower electrode layer 35, and an upperelectrode layer 37 formed on the piezoelectric layer 36.

On an interface between the Si substrate 31 and the buffer layer 32,oxygen is supplied to form an SiO₂ thermal oxide film layer 38, whichserves as a resilient board in the actuator, together with the Sisubstrate 31. For the Si substrate 31, a substrate in which the naturaloxide film is not removed was used. In the first buffer layer 32, YSZ isepitaxially grown in the cubic (100) orientation to a thickness of 5 nm.In the second buffer layer 33, CeO₂ is epitaxially grown in the cubic(100) orientation to a thickness of 10 nm. In the first oxide electrodelayer 34, YBa₂Cu₃O_(x) is epitaxially grown in the tetragonal ororthorhombic (001) orientation, to a thickness of 2 nm. In the secondoxide electrode layer 35, SrRuO₃ is epitaxially grown in thepseudo-cubic (100) orientation, to a thickness of 100 nm. In thepiezoelectric layer 36, PbZr_(0.55)Ti_(0.45)O₃ is epitaxially grown inthe rhombohedral (100) orientation. Ir is used for the upper electrodelayer 37.

The manufacturing method for the piezoelectric actuator will now bedescribed.

The washing step of the Si (100) substrate 31, the heating and warm-upstep, the deposition step of the first buffer layer 32, the depositionstep of the second buffer layer 33, the deposition step of the firstlower electrode layer 34, and the deposition step of the second lowerelectrode layer 35 are the same as in the manufacturing method for thesubstrate 100 for electronic devices shown in Example 1. After havingdeposited the lower electrode layer 35, pulsed radiation of the KrFexcimer laser is shone onto a PbZr_(0.55)Ti_(0.45)O₃ target surfacearranged opposite to the substrate, under conditions of an energydensity of 2.0 J/cm², a frequency of 10 Hz, and a pulse length of 10 ns,to generate a plasma plume consisting of Pb, Zr, Ti and O on the targetsurface. This plasma plume is irradiated onto the Si substrate 31located at a position away from the target by 40 mm, for 90 minutesunder conditions of a substrate temperature of 600° C., and an oxygenpartial pressure during deposition of 1×10⁻² Torr, so that thePbZr_(0.55)Ti_(0.45)O₃ piezoelectric layer 36 is deposited to 900 nm. Atthis time, oxygen is supplied to the interface between the Si substrate31 and the YSZ first buffer layer 32 to form an SiO₂ thermal oxide filmlayer 38 to a thickness of 500 nm.

Of the respective conditions, it is desired that the target compositionis PZT, the laser energy density is from 1.0 J/cm² to 3.0 J/cm²inclusive, the laser frequency is not lower than 5 Hz and not higherthan 15 Hz, the target-substrate distance is from 30 mm to 50 mminclusive, the substrate temperature is from 550° C. to 650° C.inclusive, and the oxygen partial pressure during deposition is from1×10⁻Torr to 1×10⁻¹ Torr inclusive. However, the respective conditionsare not limited thereto, so long as the Pb, Zr and Ti plasma can reachthe substrate at a desired constant ratio, and the epitaxial growth asPZT is possible. Moreover, similar effects can be obtained by usingferroelectrics having the perovskite structure, as represented by Pb(Mb,Nb)O₃, Pb(Zn, Nb)O₃, BaTiO₃, KNbO₃, and BiFeO₃, and solid solutions ofthese ferroelectrics and various kinds of paraelectrics, instead of PZT.

After having deposited the PZT piezoelectric layer 36, etching isperformed by using a known method such as photolithography, and thelower electrode layer 35 is taken out, and an Ir upper electrode layer37 is deposited by a known method such as sputtering, while using a maskpattern. Similar effects can be obtained by using other electrodematerials generally used for electrodes, such as Pt, instead of Ir.

The thus obtained piezoelectric capacitor 300 has an orientationrelation of Ir/PZT (100)/SrRuO₃ (100)/YBa₂Cu₃O_(x) (001)/CeO₂ (100)/YSZ(100)/Si (1.00), and an azimuthal relation of PZT <010>//SrRuO₃<010>//YBa₂Cu₃O_(x) <100>//CeO ₂ <011>//YSZ <011>//Si<011>.

As a result of measurement of the field distortion characteristic withrespect to the obtained piezoelectric actuator 300, by applying anelectric field having a frequency of 1 kHz and an amplitude of 100kV/cm, a piezoelectric constant d₃₁=200 pC/N has been obtained. Thispiezoelectric actuator has a higher field distortion characteristic as apiezoelectric actuator, as compared with d₃₁=160 pC/N of a piezoelectricactuator using a non-oriented PZT piezoelectric actuator layer. This canbe considered to be an effect of an engineered domain in which thepiezoelectric effect increases when angles between a polarizing axis ofeach domain in the piezoelectric layer and the electric field are allequal and not 0 degrees.

Moreover, if layers up to the second lower electrode layer 35 haveepitaxially grown, then in the deposition step of the piezoelectriclayer 36, any film-forming method, such as an application method by spincoating, dip coating or ink jet printing, a chemical evaporation methodsuch as MOCVD, and a physical vapor deposition method such as sputteringcan be used instead of the laser ablation, to obtain similar effects.

According to the piezoelectric actuator in which the substrate forelectronic devices having the above configuration is used for the lowerelectrode, it has been confirmed that the rhombohedral (100) epitaxialgrowth of the piezoelectric layer having the perovskite structure can beachieved, thereby enabling improvement in the field distortioncharacteristic of the piezoelectric element.

1. A substrate for electronic devices comprising: a substrate consistingof silicon and having a film-forming surface, and a first buffer layer,a second buffer layer, a first oxide electrode layer and a second oxideelectrode layer, which are grown epitaxially and laminated in this orderon the film-forming surface of the substrate, wherein said first bufferlayer is a first metal oxide having a fluorite structure, said secondbuffer layer is a second metal oxide having a fluorite structure, saidfirst oxide electrode layer is a third metal oxide having a layeredperovskite structure, and said second oxide electrode layer is a fourthmetal oxide having a simple perovskite structure.
 2. A substrate forelectronic devices according to claim 1, wherein the orientation of saidfilm-forming surface is (100), (110), or (111).
 3. A substrate forelectronic devices according to claim 1, wherein in said film-formingsurface, a diffraction pattern is not observed in a diffraction image bya RHEED method, before forming said first buffer layer.
 4. A substratefor electronic devices according to claim 1, wherein said first metaloxide is a solid solution expressed as Zr_(1-x)Mα_(x)O_(y) (0<x<1,1.5<y<2) obtained by substituting a part of Zr, being a constituentelement of zirconia, by a metal element Mα, where Mα indicates one kindof element selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er,Tm, Yb, Lu, Y, Mg, Ca, Sr and Ba, and the first metal oxide is cubicallyoriented in the (100) direction.
 5. A substrate for electronic devicesaccording to claim 1, wherein said second metal oxide is cerium oxide ora solid solution expressed as Ce_(1-x)Mβ_(x)O_(y) (0<x<1, 1.5<y<2)obtained by substituting a part of Ce, being a constituent element ofcerium oxide, by a metal element Mβ, where Mβ indicates one kind ofelement selected from Zr, La, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er,Tm, Yb, Lu, Y, Mg, Ca, Sr and Ba, and the second metal oxide iscubically oriented in the (100) direction.
 6. A substrate for electronicdevices according to claim 1, wherein said third metal oxide is a solidsolution containing a metal element My or RE as a constituent element,and expressed as MγRuO₄, RE₂NiO₄, or REBa₂Cu₃O_(x), where Mγ indicatesone kind of element selected from Ca, Sr and Ba, and RE indicates onekind of element selected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy,Ho, Er, Tm, Yb, Lu, and Y, and the third metal oxide is tetragonally ororthorhombically oriented in the (001) direction.
 7. A substrate forelectronic devices according to claim 1, wherein said fourth metal oxideis a solid solution containing a metal element Mγ or RE as a constituentelement, and expressed as MγRuO₃, (RE, Mγ)CrO₃, (RE, Mγ)MnO₃, (RE,Mγ)CoO₃, or (RE, Mγ)NiO₃, where Mγ indicates one kind of elementselected from Ca, Sr and Ba, and RE indicates one kind of elementselected from La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm, Yb,Lu, and Y, and the third metal oxide is cubically or pseudo-cubicallyoriented in the (100) direction.